Apparatus for elimination play in gear wheel transmissions

ABSTRACT

Apparatus for eliminating play between two interengaged gear wheels (4,10)f a gear wheel transmission includes a pair of disc-like transmission elements (1,2) which may be embodied in different forms. In one form of embodiment the elements (1,2) comprise two single eccentrics which are arranged movably relative to each other and so as to constitute a double eccentric. The central axis (2M) of the eccentric (2) is concentric with respect to the axis of rotation (10D) of the gear wheel (10). The spacing of the axes of rotation (4D,10D) of the two gear wheels (4,10) is a function of the eccentricity of the double eccentric (1,2). A spring (12) is connected to both eccentrics (1,2) in such a way that the two eccentrics (1,2) are pivoted relative to each other by the spring force in a direction such that the total eccentricity increases and thus play between the gear wheels (4,10), which are connected to the two eccentrics (1,2), is removed. A handle (16) is connected to a gear wheel (10) by way of an adjusting shaft (5) and the double eccentric (1,2) in such a way that, upon rotary movement in either of the two directions of rotation, said gear wheel is rotated relative to the other gear wheel (4) and at the same time an eccentric (1 or 2) is moved in the direction of slightly increasing the play between the two gear wheels (4,10).

BACKGROUND OF THE INVENTION

This invention relates to apparatus for eliminating play between twointerengaged gear wheels of a gear wheel transmission which isself-locking in relation to a moment of rotation acting on the outputside. Apparatuses of this kind can be used in particular in eccentrictransmission systems in hinge fittings of motor vehicle seats with anadjustable backrest, but they can also be used in drafting machines andin other applications.

Eccentric transmission systems are known from DE No. 2 734 568A. Theyare used in many situations. In many cases it is desirable to eliminatethe play which is present in known eccentric transmission systems.

Previously known apparatuses for eliminating the play betweeninterengaging gear wheels of a gear wheel transmission are expensive andare therefore unsuitable for many purposes, for example for use in agear wheel transmission system for adjusting the inclination of thebackrest of a motor vehicle seat.

SUMMARY OF THE INVENTION

It is an object of the invention to eliminate the play in a gear wheeltransmission by simple means which can be comparatively simple producedin a compact design, and can thus be used in many cases in which thepreviously known apparatuses cannot be employed.

According to the present invention there is provided apparatus foreliminating play between two interengaged gear wheels of a gear wheeltransmission which is self-locking in relation to a moment of rotationacting on the output side, characterised by the combination of thefollowing features:

(a) the central axis of one disc-like transmission element is concentricwith respect to the axis of rotation of one gear wheel;

(b) the spacing of the axes of rotation of the two gear wheels is afunction of the rotation of the disc-like transmission elements relativeto each other;

(c) a spring is connected to both disc-like transmission elements insuch a way that the two disc-like transmission elements are pivotedrelative to each other by the spring force in a direction, such that thedisplacement of the central axes of the disc-like transmission elementsrelative to each other in radial direction increases and thus playbetween the gear wheels, which are connected to the disc-liketransmission elements, is removed; and

(d) a handle is connected to a gear wheel by way of an adjusting shaftand the disc-like transmission elements in such a way that, upon rotarymovement in either of the two directions of rotation, said gear wheel isrotated relative to the other gear wheel and at the same time adisc-like transmission element is moved in the direction of slightlyincreasing the play between the two gear wheels.

This arrangement ensures that, in the event of possible incipientjamming of the gear wheels, by taking advantage of the friction betweenparts which are moved, a small amount of play can be produced betweenthe gear wheels and thus any jamming can be avoided.

In many cases it is advantageous to employ in embodiment of theinvention in which the disc-like transmission elements are in the formof two single eccentrics which are arranged movably relative to eachother and which are arranged as a double eccentric, means are providedwhich cause the spacing of the axes of rotation of the two gear wheelsto be a function of the respective resulting eccentricity of the doubleeccentric, and the force of the spring element causes the two eccentricsto be pivoted relative to each other in such a direction that the totaleccentricity increases and thus the play between the gear wheels, whichare connected to the two eccentrics, is removed. However, to simplifyproduction, it may be useful not to provide a double eccentric but toemploy an embodiment in which the disc-like transmission elements are inthe form of a first guide disc which is displaceable on the adjustingshaft, which shaft is concentric with respect to the central axis of theone gear wheel, in a radial direction relative to said adjusting shaft,and a second guide disc which is connected to the first guide disc insuch a way that, in the event of relative rotation therebetween, thefirst guide disc is displaced radially relative to the adjusting shaft.

The invention will now be further described, by way of illustrative andnon-limiting example, with reference to the accompanying drawings, whichare partly in diagrammatically simplified form. Corresponding componentsare denoted by the same reference numerals throughout the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 show the principles of operation of embodiments of theinvention, more particularly

FIG. 1 shows an embodiment (vario-eccentric) with two interengaged andrelatively pivotal part eccentrics 1,2,

FIG. 2 shows the system of FIG. 1, but in the condition of movement, and

FIG. 3 shows the vario-eccentric in three different positions ofeccentricity, namely zero eccentricity (FIG. 3a), medium eccentricity(FIG. 3b) and maximum eccentricity (FIG. 3c);

FIG. 4 shows use of the above embodiment in a motor vehicle seat;

FIG. 5 is an exploded view of the components of the above embodiment;

FIGS. 6a to 6c show an advantageous alternative form of the embodimentof FIG. 5, with only a small number of individual components which aredifferent from each other,

FIG. 6a showing a sectional view in highly simplified diagrammatic form,

FIG. 6b showing a sectional view of a possible technical construction ofthe alternative form shown in FIG. 6a, and

FIG. 6c showing an exploded view of this alternative form;

FIG. 7 shows an exploded view of another embodiment;

FIG. 8 shows an exploded view of another embodiment wherein, instead ofeccentric discs, other guide discs are used; and

FIG. 9 shows a sectional view taken perpendicularly to the axis ofrotation, through the guide discs shown in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments shown in FIGS. 1 to 7 all comprise disc-liketransmission elements in the form of eccentrics 1,2.

In the embodiment shown in FIGS. 1,2 and 5 (vario-eccentric), an innereccentric or part eccentric 1 having a central axis 1M and an axis ofrotation 1D is provided with a bore by means of which it is carriedslidably on a sleeve 3 which is connected to an outwardly disposedinternally toothed gear wheel 4. An adjusting shaft 5 is mounted in thesleeve 3. The axes of symmetry of the sleeve 3, the outwardly disposedgear wheel 4 and the adjusting shaft 5 coincide at a point 6. An outereccentric or part eccentric 2, having a central axis 2M and an axis ofrotation 2D, is mounted so that an inner surface 7 thereof slides on theinner eccentric 1 and an outer surface 8 thereof slides in a bore 10B ofthe same diameter, which bore is disposed centrally (axis of symmetry 9)within a spur gear wheel 10 which is in mesh with the gear wheel 4 andis smaller than the gear wheel 4. The central axes 2M of the eccentric 2is concentric with respect to the axes of rotation (9,10D) of the gearwheel 10. The spacing of the axes of rotation 4D,10D of the two gearwheels 4,10 is a function of the respective resulting eccentricity ofthe double eccentric 1,2. Rotation of the adjusting shaft 5 istransmitted to both the eccentrics 1,2, by way of an entrainment disc 11(shown only schematically in FIGS. 1 and 2). Rotation of the eccentrics1,2 relative to each other causes a change in the total eccentricity,which is additively composed of the two partial eccentricities resultingfrom the position of the part eccentrics 1,2 relative to each other.FIGS. 3a to 3c illustrate the total eccentricity, which changes withrotation of the part eccentrics 1,2 relative to each other. The twoeccentrics 1,2 are connected together by a spring 12 (shown onlyschematically in FIGS. 1 and 2) in such a way that the eccentrics arepivoted relative to each other by the spring force in a direction suchthat the total eccentricity tends to increase.

When the adjusting shaft 5 is not actuated, in each instantaneous gearwheel position, the double eccentric 1,2 is of such an eccentricity, dueto the effect of the spring 12, that both gear wheels 4,10 are pressedinto engagement with each other without play (FIG. 1). This condition ismaintained even when an externally acting force seeks to rotate the twotoothed rings (4,10) relative to each other, as the double eccentric 1,2sticks in the position in which the components are pressed together.This sticking is caused by the friction between the outer eccentricsliding surface 8 and the inside surface of the bore in the gear wheel10, and between the sleeve 3 and the inside surface of the bore in theinner eccentric 1.

The double eccentric 1,2 is released from the position in which thecomponents thereof are pressed against each other, when the adjustingshaft 5 begins to rotate, by a slight rotary movement of the parteccentric 1 or 2 respectively, in the direction of reducing the totaldegree of eccentricity (FIG. 2). The rotary movement in this directionis produced by the arrangement of entrainment pins 13',13 on theeccentrics 1,2 and the shape of the entrainment disc 11. The entrainmentpins 13,13' and recesses 14,14' in the entrainment disc 11 are soarranged that, upon rotary movement of the adjusting shaft 5, only theentrainment pin of one eccentric is respectively engaged by theentrainment disc 11, while the entrainment pin of the other eccentricretains a certain clearance in the associated recess in the entrainmentdisc. The eccentric, which is driven by way of a hand wheel 16, theadjusting shaft 5 and the entrainment disc 11, transmits its rotarymovement by way of the spring 12 to the second eccentric which, due tofriction between the sliding surfaces, lags in its rotary movementslightly behind the driven eccentric. This causes a small amount ofradial displacement of the spur gear 10 in an inward direction and thusproduces a certain (very small) amount of play between the sides of theteeth of the gear wheels 4,10, which leads to the two gear wheels 4,10being released from the condition in which they are pressed against eachother, and permits the adjusting shaft 5 to rotate easily.

During rotation of the adjusting shaft 5, by virtue of the slidingfriction (instead of the static friction as above) between the firsteccentric 1 and the sleeve 3 or between the second eccentric 2 and thebore 10B in the spur gear 10, the motion is controlled in such a waythat, in the event of jamming of the gear wheels due to productiontolerances, a small amount of play between the sides of the gear teethis maintained. When jamming begins to occur, the sliding frictionbetween the above-mentioned surfaces increases greatly and, as alreadydescribed above with regard to the operation at the beginning of therotary movement, causes a slight radial displacement of the spur gear 10in an inward direction. On the other hand, in the event of playbeginning to become too great, the force of the spring 12 between thetwo eccentrics 1,2 causes the respective non-driven eccentric to performa take-up rotary motion and the spur gear 10 is thereby displacedslightly radially outwardly so that the play between the sides of theteeth is adjusted to a minimum value.

Thus, the non-driven eccentric lags in its rotary movement behind thedriven eccentric precisely only to such an extent as is required bymanufacturing tolerances.

Therefore, the vario-eccentric compensates automatically, by changingits degree of eccentricity, both for any manufacturing tolerances andalso any dimensional changes due to wear.

To understand more fully the operation of the above-describedentrainment operation, it should be understood that the recesses 14,14'in the entrainment disc 11 are each of greater extent, in the peripheraldirection, than the associated pin 13,13' and the pin is arranged in therecess movably with play in the peripheral direction. When a gear wheel4 or 10 is rotated, a pin 13 or 13' lies against the end of theassociated recess 14 or 14' and the length of the other recess 14' or 14and the force of the spring 12 are such that the other pin 13' or 13 isalso moved in the peripheral direction. In fact, the lengths (in theperipheral direction) of the two recesses 14,14' and the force of thespring 12 are such that, in one direction of rotation, a pin (13 or 13')lies against one end of one recess (14 or 14'), while in the oppositedirection of rotation the other pin (13' or 13) lies against theopposite end of the other recess (14' or 14).

In order to prevent the inner eccentric 1 from jamming with respect tothe outer eccentric 2, a rolling bearing 15 is provided between the twoeccentrics. The recesses 14,14' in the entrainment disc 11 are limitedto a certain angular extent in order to ensure that the spring 12 actingbetween the two eccentrics 1,2 cannot be overstressed by external forcesacting thereon.

When there is a change in the direction of rotation, the function of thedriven eccentric and the eccentric, which is entrained by the springaction, is also interchanged. This provides that the same controlmechanisms come into operation automatically when the adjusting shaft 5is rotated, irrespective of the direction of rotation.

After the adjusting operation is concluded, the spring force acting onthe two eccentrics 1,2 causes the components automatically the return tothe condition of being pressed together, thereby ensuring that theeccentric transmission assembly is free from play.

It is advantageous in the above-described eccentric mechanism for thedegrees of eccentricity of both part eccentrics 1,2 to be made of equalmagnitude. If, by a suitable adjusting mechanism, both eccentrics arebrought into an opposed position (FIG. 3a), the two part eccentricitiesare added together to give a total eccentricity of zero. This means thatboth toothed rings (gear wheels) 4,10 are entirely disengaged from eachother, thereby permitting the inclination of a seat backrest (e.g. of amotor vehicle seat) to which the mechanism is fitted (FIG. 4) to beeasily adjusted over wide ranges of angular movement. After this coarseadjustment has been concluded, adjustment of the angular position may becontinuously set without play, by re-engaging the two toothed rings 4,10with each other. The backrest of the seat, in this arrangement, isnon-rotatably fixedly connected to one of the gear wheels 4,10 and theother gear wheel is non-rotatably mounted at a position fixed withrespect to the vehicle floor. For example, as shown, one of the gearwheels 4,10 of the transmission is connected by way of a securing arm4BF or 10BF to the backrest of the seat, while the second gear wheel 10or 4 is connected to the bottom portion of the seat by way of a securingarm 10BF or 4BF of the same kind. When the hand wheel 16 is actuated,the two gear wheels are caused to roll against each other, therebyeffecting adjustment of the angle of inclination of the backrest.

Preferably, the maximum possible degree of eccentricity of the doubleeccentric 1,2 is at least equal to, and preferably greater than, thedifference in the radii of the two pitch circles of the two gear wheels4,10.

FIGS. 6a to 6c show an advantageous alternative form of the embodimentshown in FIGS. 1,2 and 5, wherein the rolling bearing is unnecessary byvirtue of suitable selection of the geometrical dimensions of theeccentrics. For greater clarity of the drawing, it is assumed that thetwo gear wheels 4 and 10 are fully engaged with each other, at themaximum possible degree of eccentricity of the double eccentric 1,2(FIG. 3c: all axes of the eccentrics lying in one plane). The innereccentric 1 slides in a bore of the same diameter in the gear wheel withthe internally toothed ring 4, the central axis 1M of the eccentric 1and the axis 4D of the gear wheel 4 coinciding. The outer eccentric 2slides in a bore of the same diameter in the gear wheel 10; the centralaxis 2M of the second eccentric and the axis 10D of the gear wheel 10also coincide. The diameters of the two eccentrics 1,2 are equal. Botheccentrics are connected together either by a sleeve 3' formed aroundthe axis of rotation 2D of one eccentric 2, with the other eccentric 1mounted rotatably about the sleeve (FIG. 6a) or by the adjusting shaft 5(FIGS. 6b and 6c).

The embodiment of FIGS. 6a to 6c is distinguished by low productioncosts since both the eccentrics 1,2 and both entrainment discs 11,11'can each be totally identical to the other and the rolling bearing 15 ofthe preceding embodiment is omitted.

FIG. 7 shows an embodiment wherein the rolling bearing 15 becomesunnecessary because the smaller eccentric 1 is fixedly connected to acircular disc 17 whose diameter corresponds to that of the secondeccentric 2. As in the embodiment of FIGS. 6a to 6c, the eccentric 2 andthe circular disc member 17 of the eccentric 1 are received byconcentric bores 4B and 10B respectively in the gear wheels 4 (4P) and10 respectively. The selected dimensions ensure that, during therotating operation, the frictional moments which occur in the bores ofthe gear wheels 4,10 and which are required to keep the play at aminimum are greater than the moment of the frictional force between thesurfaces 1A and 7 of the two eccentrics. The transmission system isdriven by way of two entrainment discs 11,11', which are fixedlyconnected to the adjusting shaft 5 and entrainment pins 13,13' of theeccentrics 2,1, which pins engage into the recesses 14,14' in theentrainment discs.

In the embodiment shown in FIGS. 8 and 9, a first circular disc 1 with aspirally extending recess 1S is arranged rotatably in a concentric bore4B in a plate 4P which is connected to the internally toothed gear wheel4. The disc 1 in turn has a concentric bore 1B for receiving theadjusting shaft 5. The spur gear 10, which is disposed eccentricallywith respect to the internally toothed gear wheel 4, has a bore 10Bwhich is concentric with respect to the spur gear tooth ring, forreceiving a second circular disc 2 with a radial slot 2S. Fixedlyconnected to the disc 2 is a guide or mounting pin 18 which engages intothe spiral recess 1S in the first disc 1.

(For the sake of simplicity of language, the two discs 1 and 2 with theradial guide surface and the spiral guide surface respectively arejointly referred to hereinafter as `guide discs`).

The distance of the pin 18 from the central axis 2M of the guide disc 2is such that, after the adjusting shaft 5 has been fitted into thecentral recesses 1B and 2B of the guide discs 1,2 with the spring 12being omitted, both gear wheels 4 and 10 would mesh with play. The forceof the spring 12 causes both guide discs 1,2 to rotate relative to eachother. In this event the pin 18 which is connected to the one guide disc2 slides along the guide surface 19 of the spirally extending slot 1S ofthe other guide disc 1 and displaces the guide disc 2, in which the pinis secured, and the gear wheel 10, which is arranged concentrically withrespect to said guide disc, in the radial direction. The path of thespiral is such that, when the two guide discs 1,2 rotate relative toeach other under the effect of the force of the spring, the gear wheel10 is displaced outwardly in the direction of the point of engagement ofthe two gear wheels, until there is play-free engagement between the twosets of teeth. As in the embodiments of FIGS. 6 and 7, the transmissionmeans is driven by way of two entrainment discs 11 and 11' which arefixedly connected to the adjusting shaft 5.

During the rotary movement of the adjusting shaft 5, when jamming tendsto begin, the respective guide disc 2 or 1 which is not engaged by theentrainment discs 11 and 11' respectively lags slightly behind in therotary movement, by virtue of sliding friction, relative to the guidedisc 1 or 2 which is driven by the other entrainment disc 11' and 11,respectively. In this event, the pin 18 which is connected to the oneguide disc 2 slides along the guide surface 19 of the spiral slot 1S inthe other guide disc 1 in such a direction as to produce radialdisplacement of the spur gear 10 inwardly, until the `threatening`jamming is eliminated.

With regard to the mode of operation of this embodiment, it does notmatter if, unlike FIG. 8, the guide disc is fitted into the respectiveother gear wheels. It also does not matter if the spiral and the radialrecess are provided in one disc and the central bore and the guide pinare provided in the other disc.

An advantageous alternative form of this embodiment is produced if thepart of the guide pin which projects into the guide slot in the one discis surrounded by a ball bearing assembly for reducing friction.

Another advantageous alternative form is produced by the guide pin beingmounted rotatably in its guide disc and by the part of the pin whichengages into the guide slot in the other guide disc being adapted to theshape of the sides of the slot.

For many purposes, it is advantageous for the radial slot 2S in theguide disc 2 to be longer than the depth of the teeth of theinterengaging gear wheels. If the two disc-like transmission elements1,2 are moved into a concentric position relative to each other, bothgear wheels 4 and 10 are brought entirely out of engagement therebypermitting rapid adjustment of the angle of inclination of the backrestof the seat, over wide ranges of angular movement. When this coarseadjustment is concluded, adjustment of the angle of inclination may becontinuously set again without play, by reengaging the two toothed ringswith each other.

I claim:
 1. Apparatus for eliminating play between two interengaged gearwheels (4, 10) of a gear wheel transmission which is self-locking inrelation to a moment of rotation acting on the output side and includestwo disc-like transmission elements (1, 2) each associated with therespective gear wheel, characterized by the combination of the followingfeatures:(a) the central axis (2M or 1M) of each of the disc-liketransmission elements (2 or 1) is concentric with respect to the axis ofrotation (10D or 4D) of the respective gear wheel (10 or 4); (b) thespacing of the axes of rotation (4D, 10D) of the two gear wheels (4, 10)is a function of the rotation of the disc-like transmission elements (1,2) relative to each other; (c) a spring (12) is connected to bothdisc-like transmission elements (1, 2) in such a way that the twodisc-like transmission elements (1, 2) are pivoted relative to eachother by the spring force in a direction, such that the displacement ofthe central axes (1M, 2M) of the disc-like transmission elementsrelative to each other in radial direction increases and thus playbetween the gear wheels (4, 10), which are connected to the respectivedisc-like transmission elements (1, 2), is removed; and (d) a handle(16) is connected to one of said gear wheels (10) by way of an adjustingshaft (5) and the disc-like transmission elements (1, 2) in such a waythat, upon rotary movement in either of the two directions of rotation,said one gear wheel is rotated relative to the other gear wheel (4) andat the same time the respective disc-like transmission element (1, 2) ismoved in the direction of slightly increasing the play between the twogear wheels (4, 10).
 2. Apparatus according to claim 1, wherein thedisc-like transmission elements (1,2) are in the form of two singleeccentrics which are arranged movably relative to each other and whichare arranged as a double eccentric, means are provided which cause thespacing of the axes of rotation (4D,10D) of the two gear wheels (4,10)to be a function of the respective resulting eccentricity of the doubleeccentric (1,2), and the force of the spring element (12) causes the twoeccentrics (1,2) to be pivoted relative to each other in such adirection that the total eccentricity increases and thus the playbetween the gear wheels (4,10), which are connected to the twoeccentrics (1,2), is removed.
 3. Apparatus according to claim 2, whereinthe axis of rotation (1D) of one eccentric (1) of said two eccentrics isconcentric with respect to the axis of rotation (4D) of said other gearwheel (4).
 4. Apparatus according to claim 3, wherein the two eccentrics(1, 2) are fitted one into the other and are pivotal relative to eachother.
 5. Apparatus according to claim 4, including a bearing (15)arranged between the two eccentrics (1,2), the bearing (15) having alower degree of frictional resistance than the friction between asliding surface of one eccentric (1 or 2) and one of the gear wheels (4or 10) and also less than the friction between the other gear wheel (10or 4) and an associated sliding surface of the other eccentric (2 or 1).6. Apparatus according to claim 2, wherein the central axis (1M or 2M)of each of said two eccentrics (1 or 2) is concentric with respect tothe axis of rotation (4D or 10D) of the respective gear wheel (4 or 10).7. Apparatus according to claim 2, wherein the gear wheel transmissioncomprises a larger gear wheel (4) provided with internal teeth and asmaller gear wheel (10) which meshes with the gear wheel (4) and whichis provided with external teeth, and the maximum possible degree ofeccentricity of the double eccentric (1,2) is at least equal to thedifference in the radii of the two pitch circles of the two gear wheels(10,4).
 8. Apparatus according to claim 7, wherein the degrees ofeccentricity of the two single eccentrics (1,2) are of equal magnitude.9. Apparatus according to claim 1, wherein the disc-like transmissionelements are in the form of a first guide disc (2) which is displaceableon the adjusting shaft (5), which shaft is concentric with respect tothe central axis of the other gear wheel (4), in a radial directionrelative to said adjusting shaft, and a second guide disc (1) which isconnected to the first guide disc in such a way that, in the event ofrelative rotation therebetween, the first guide disc (2) is displacedradially relative to the adjusting shaft (5).
 10. Apparatus according toclaim 9, wherein the second guide disc (1) has a guide surface (19)which extends eccentrically with respect to the axis of rotation thereofand against which lies a connecting member (18) which is fixedlyconnected to the first guide disc (2).
 11. Apparatus according to claim10, wherein the guide surface (19) which extends eccentrically withrespect to the axis of rotation thereof is in the form of a spiral. 12.Apparatus according to claim 11, wherein the connecting member (18)which lies against the guide surface (19) is a pin rotatably mounted inthe first guide disc (2) and has a head portion which is adapted to theshape of the sides of the guide surface of the second guide disc (1).13. Apparatus according to claim 12, wherein the guide surface (19) isin the form of the side of a slot-shaped recess provided in one of theguide discs (1, 2).
 14. Apparatus according to claim 13, wherein the pin(18) serving as a connecting element is rotatably mounted in the firstguide disc (2) by means of a ball bearing assembly.
 15. Apparatusaccording to claim 13, wherein the pin (18) serving as a connectingelement is rotatably mounted by means of a ball bearing assemblyprovided between a head portion of the pin and the guide surface of thesecond guide disc (1).
 16. Apparatus according to claim 9, wherein thegear wheel transmission comprises a larger gear wheel (4) provided withinternal teeth and a smaller gear wheel (10) which engages with the gearwheel (4) and which is provided with external teeth, and the maximumoperative range of movement of the two guide discs (1,2) relative toeach other in the radial direction is at least equal to the differencebetween the radii of the two pitch circles of the two gear wheels(4,10).
 17. Apparatus according to claim 16, wherein the maximumoperative range of movement of the two guide discs (1,2) relative toeach other in the radial direction is of such a magnitude that, bymovement in the direction of an increase in the play, the twointerengaging gear wheels (4,10) can be brought out of engagement. 18.Apparatus according to claim 1, further comprising an entrainment devicewhich comprises entrainment means (11, 11') and pins (13, 13') whichengage into recesses provided in the disc-like transmission elements (1,2) and secured to the entrainment means.
 19. Apparatus according toclaim 1, comprising an entrainment device including entrainment means(11, 11') and pins (13, 13') engaged into respective recesses (14, 14')provided in the entrainment means (11, 11') and secured to therespective disc-like transmission elements.
 20. Apparatus according toclaim 19, wherein the entrainment means (11) is non-rotatably connectedto the handle (16) and has one of said recesses (14 or 14') into whichthe respective pin (13 or 13') which is fixedly connected to therespective disc-like transmission element (2 or 1) engages in such a waythat, upon rotary movement of the handle (16), said respective disc-liketransmission element is pivoted and therewith a rolling movement of oneof said gear wheels (4 or 10) on the other gear wheel (10 or 4) isinitiated.
 21. Apparatus according to claim 19, wherein the entrainmentmeans (11) has two recesses (14, 14') and a respective pin (13, 13')engages into a respective recess pin (13, 13') being fixedly connectedto a respective disc-like transmission element (2, 1).
 22. Apparatusaccording to claim 21, wherein the two recesses (14, 14') in theentrainment means (11) are each of greater extent, in the peripheraldirection with respect to the axis of the entrainment means, than theassociated pin (13 or 13') and the respective pin is arranged in therespective recess (14 or 14') movably with play in the peripheraldirection.
 23. Apparatus according to claim 22, wherein, when theadjusting shaft (5) is rotated, the respective pin (13 or 13') liesagainst the end of the associated recess (14 or 14') of the entrainmentmeans (11) and the length of the other recess (14' or 14) and the forceof the spring (12) are such that the other pin (13' or 13) is also movedin the peripheral direction.
 24. Apparatus according to claim 23,wherein the length, in the peripheral direction, of each of the tworecesses (14, 14') and the force of the spring (12) are such that, inone direction of rotation, the respective pin (13 or 13') lies againstone end of the associated recess (14, 14'), while in the oppositedirection of rotation the other pin (13' or 13) lies against theopposite end of the other recess (14' or 14).
 25. Apparatus according toclaim 24, wherein the disc-like transmission elements (1,2) are disposedbetween the entrainment means (11,11'), which are of a disc-likeconfiguration.
 26. Apparatus according to claim 1, which is formed as anadjusting means for adjusting the inclination of the backrest of a motorvehicle seat, wherein the backrest of the seat is non-rotatably fixedlyconnected to one of the gear wheels (4 or 10) of the gear wheeltransmission and the other gear wheel (10 or 4) is non-rotatably mountedat a position fixed with respect to the vehicle floor.
 27. Apparatusaccording to claim 26, wherein the other gear wheel is mounted to thebottom portion of the seat.